Electronics Fundamentals Circuits, Devices, and Applications Floyd Chapter 16

Summary Semiconductors Semiconductors are crystalline materials that are characterized by specific energy bands for electrons. Between the bands are gaps; these gaps represent energies that electrons cannot posses. Energy Conduction band The last energy band is the conduction band, where electrons are mobile. Energy gap Valence band Energy gap The next to the last band is the valence band, which is the energy level associated with electrons involved in bonding. Second band Energy gap First band Nucleus

Summary Electron and hole current At room temperature, some electrons have enough energy to jump into the conduction band. After jumping the gap, these electrons are free to drift throughout the material and form electron current when a voltage is applied. Electron-hole pair Energy For every electron in the conduction band, a hole is left behind in the valence band. Conduction band Heat energy Energy gap Valence band

Summary Electron and hole current The electrons in the conduction band and the holes in the valence band are the charge carriers. In other words, current in the conduction band is by electrons; current in the valence band is by holes. When an electron jumps to the conduction band, valence electrons move from hole-to-hole in the valence band, effectively creating “hole current” shown by gray arrows. Free electron Si Si Si

Summary Impurities By adding certain impurities to pure (intrinsic) silicon, more holes or more electrons can be produced within the crystal. To increase the number of conduction band electrons, pentavalent impurities are added, forming an n-type semiconductor. These are elements to the right of Si on the Periodic Table. To increase the number of holes, trivalent impurities are added, forming a p-type semiconductor. These are elements to the left of Si on the Periodic Table.

Summary The pn junction diode When a pn junction is formed, electrons in the n-material diffuse across the junction and recombine with holes in the p-material. This action continues until the voltage of the barrier repels further diffusion. Further diffusion across the barrier requires the application of a voltage. The pn junction is basically a diode, which is a device that allows current in only one direction. A few typical diodes are shown.

Summary Forward bias When a pn junction is forward-biased, current is permitted. The bias voltage pushes conduction-band electrons in the n-region and holes in the p-region toward the junction where they combine. p-region n-region The barrier potential in the depletion region must be overcome in order for the external source to cause current. For a silicon diode, this is about 0.7 V. p n R + - VBIAS The forward-bias causes the depletion region to be narrow.

Summary Reverse bias When a pn junction is reverse-biased, the bias voltage moves conduction-band electrons and holes away from the junction, so current is prevented. p-region n-region The diode effectively acts as an insulator. A relatively few electrons manage to diffuse across the junction, creating only a tiny reverse current. p n R - + VBIAS The reverse-bias causes the depletion region to widen.

Summary Diode characteristics The forward and reverse characteristics are shown on a V-I characteristic curve. IF In the forward bias region, current increases dramatically after the barrier potential (0.7 V for Si) is reached. The voltage across the diode remains approximately equal to the barrier potential. Forward bias VBR (breakdown) VR VF 0.7 V Barrier potential Reverse bias The reverse-biased diode effectively acts as an insulator until breakdown is reached. IR

Summary Diode models The characteristic curve for a diode can be approximated by various models of diode behavior. The model you will use depends on your requirements. IF The ideal model assumes the diode is either an open or closed switch. Forward bias The practical model includes the barrier voltage in the approximation. VR VF 0.7 V Reverse bias The complete model includes the forward resistance of the diode. IR

Summary Half-wave Rectifier Rectifiers are circuits that convert ac to dc. Special diodes, called rectifier diodes, are designed to handle the higher current requirements in these circuits. D The half-wave rectifier converts ac to pulsating dc by acting as a closed switch during the positive alteration. + - RL D - + The diode acts as an open switch during the negative alteration. RL

Summary Full-wave Rectifier The full-wave rectifier allows unidirectional current on both alterations of the input. The center-tapped full-wave rectifier uses two diodes and a center-tapped transformer. The ac on each side of the center-tap is ½ of the total secondary voltage. Only one diode will be biased on at a time. F D1 Vsec 2 Vsec 2 RL D2

Summary Bridge Rectifier The bridge rectifier is a type of full-wave circuit that uses four diodes. The bridge rectifier does not require a center-tapped transformer. At any instant, two of the diodes are conducting and two are off. F D3 D1 RL D2 D4

Summary Power supplies By adding a filter and regulator to the basic rectifier, a basic power supply is formed. Typically, a large electrolytic capacitor is used as a filter before the regulator, with a smaller one following the regulator to complete filtering action. IC regulator F D3 D1 7805 D2 D4 C1 C2 1000 mF 1 mF

Summary Special-purpose diodes Special purpose diodes include Zener diodes – used for establishing a reference voltage Varactor diodes – used as variable capacitors Light-emitting diodes – used in displays Photodiodes – used as light sensors

Selected Key Terms PN junction Diode The boundary between n-type and p-type semiconductive materials. An electronic device that permits current in only one direction.

Selected Key Terms Barrier potential Forward bias Reverse bias Full-wave rectifier The inherent voltage across the depletion region of a pn junction diode. The condition in which a diode conducts current. The condition in which a diode prevents current. A circuit that converts an alternating sine-wave into a pulsating dc consisting of both halves of a sine wave for each input cycle.

Selected Key Terms Bridge rectifier Zener diode Varactor Photodiode A type of full-wave rectifier consisting of diodes arranged in a four corner configuration. A type of diode that operates in reverse breakdown (called zener breakdown) to provide a voltage reference. A diode used as a voltage-variable capacitor. A diode whose reverse resistance changes with incident light.

Quiz 1. An energy level in a semiconductor crystal in which electrons are mobile is called the a. barrier potential. b. energy band. c. conduction band. d. valence band.

Quiz 2. A intrinsic silicon crystal is a poor conductor of electricity. an n-type of material. a p-type of material. an excellent conductor of electricity.

Quiz 5. The breakdown voltage for a silicon diode is reached when a. the forward bias is 0.7 V. b. the forward current is greater than 1 A. c. the reverse bias is 0.7 V. d. none of the above.

Quiz 6. The circuit shown is a a. half-wave rectifier. b. full-wave rectifier. c. bridge rectifier. d. zener regulator.

Quiz 7. PIV stands for a. Positive Ion Value. b. Programmable Input Varactor. c. Peak Inverse Voltage. d. Primary Input Voltage.

Quiz 8. A type of diode used a a voltage-variable capacitor is a a. varactor. b. zener. c. rectifier. d. LED.

Quiz 9. If one of the four diodes in a bridge rectifier is open, the output will a. be zero. b. have ½ as many pulses as normal. c. have ¼ as many pulses as normal. d. be unaffected.